1. Field of the Invention
The present invention relates to a graphene-graphene fused material, and more particularly, to a method of preparing a graphene-graphene fused material and a method of preparing a graphene-substrate composite using the graphene-graphene fused material, which may embody a substrate having superior performances in thermal conductivity, electrical conductivity and mechanical strength by improving the connectivity and bonding strength between graphenes and dispersibility of a fused material in the substrate.
2. Discussion of Related Art
Graphene is a 2-dimensional nano sheet having a honeycomb lattice made of sp2-bonded carbon atoms, and has a high usability as a negative electrode active material of a lithium secondary battery and an electrode active material of an ultra-high capacity capacitor due to a high specific surface area and superior electrical conductivity and mechanical strength. FIG. 1 is a structural view illustrating a 2-D nanosheet of single layer graphene. As illustrated in FIG. 1, the graphene is made of a 2-dimensional single layer nanosheet. Such graphene has emerged as a future core material of the material industry.
For application of graphene to a large-sized graphene sheet, a preparation method using a chemical vapor deposition process has been proposed, but has a problem in that the large-sized graphene sheet may be formed only on a copper substrate, the manufacturing itself is difficult, or economic feasibility and productivity are poor.
Accordingly, a method of preparing graphene as powder and mixing the powder with a polymer substrate or the like to prepare a large-sized sheet has been proposed, but dispersibility is not good when graphene powder is mixed with the substrate, and thus graphene powder is dispersed in an agglomerated state. This problem is more frequently encountered in reduced graphene, particularly. When graphene oxide is used in order to prevent the problem, the selection of the type of graphene to be used is limited and thermal conductivity, electrical conductivity and the like are reduced due to the use of graphene oxide.
Further, a slight floating portion is present between the interface in which graphene and a substrate contact each other according to the type of the substrate, and thus electrical conductivity, thermal conductivity, gas barrier properties or the like are remarkably degraded due to decreased interfacial properties.
In order to address the issue, a method of providing various types of functional groups, such as —COOH, —COO−, —OH, —NH, or the like at corners of nano graphene according to the type of the substrate material as illustrated in FIG. 2 has been suggested. The process of providing functional groups at the corners of nano graphene is generally conducted in a liquid, such as an acid or alkali, but increases subsequent processes to deteriorate economic feasibility and productivity. Moreover, there is a limit on the improvement of dispersibility of graphene although functional groups are provided.
Meanwhile, a substrate material is still present in a space between graphene powders even when dispersibility of graphene is greatly improved, and when the substrate material has poor thermal and electrical properties as compared to graphene, the prepared sheet fails to achieve the desired level of thermal and electrical properties. Furthermore, when a separation distance between graphenes is reduced by greatly increasing the content of graphene powder in the sheet so as to resolve this problem, sheet formability is degraded, and mechanical strength of the sheet is decreased.
Therefore, research and development on the substrate which has enhanced performances in thermal conductivity and electrical conductivity by increasing connectivity, bonding strength and dispersibility of graphenes in the substrate is urgently needed.